ARMAttribute: fix access to structs, stream and nul (#203)

ARMAttribute: fix access to structs, stream and nul (#203)

The __init__ function of ARMAttribute has two parameters
structs and stream through which the caller can pass in the
relevant objects (ARMAttributesSubsubsection does that after
seeking to the right position in stream).

The accesses for TAG_SECTION and TAG_SYMBOL, however, were
referring to non-existing members instead of the parameters.

Additionally, one assertion tries to access an undefined
'null_byte' variable which should be 'nul' instead.